Strings Genome 559: Introduction to Statistical and Computational Genomics Prof. William Stafford Noble

Strings A string is a sequence of letters (called characters). In Python, strings start and end with single or double quotes. >>> “foo” ‘foo’ >>> ‘foo’

Defining strings Each string is stored in the computer’s memory as a list of characters. >>> myString = “GATTACA” myString

Accessing single characters You can access individual characters by using indices in square brackets. >>> myString = “GATTACA” >>> myString[0] ‘G’ >>> myString[1] ‘A’ >>> myString[-1] >>> myString[-2] ‘C’ >>> myString[7] Traceback (most recent call last): File "<stdin>", line 1, in ? IndexError: string index out of range Negative indices start at the end of the string and move left.

Accessing substrings >>> myString = “GATTACA”

Special characters Escape sequence Meaning \\ Backslash \’ The backslash is used to introduce a special character. >>> "He said, "Wow!"" File "<stdin>", line 1 "He said, "Wow!"" ^ SyntaxError: invalid syntax >>> "He said, 'Wow!'" "He said, 'Wow!'" >>> "He said, \"Wow!\"" 'He said, "Wow!"' Escape sequence Meaning \\ Backslash \’ Single quote \” Double quote \n Newline \t Tab

More string functionality >>> len(“GATTACA”) 7 >>> “GAT” + “TACA” ‘GATTACA’ >>> “A” * 10 ‘AAAAAAAAAA >>> “GAT” in “GATTACA” True >>> “AGT” in “GATTACA” False Length Concatenation Repeat Substring test

String methods In Python, a method is a function that is defined with respect to a particular object. The syntax is <object>.<method>(<parameters>) >>> dna = “ACGT” >>> dna.find(“T”) 3

String methods >>> "GATTACA".find("ATT") 1 >>> "GATTACA".count("T") 2 >>> "GATTACA".lower() 'gattaca' >>> "gattaca".upper() 'GATTACA' >>> "GATTACA".replace("G", "U") 'UATTACA‘ >>> "GATTACA".replace("C", "U") 'GATTAUA' >>> "GATTACA".replace("AT", "**") 'G**TACA' >>> "GATTACA".startswith("G") True >>> "GATTACA".startswith("g") False

Strings are immutable Strings cannot be modified; instead, create a new one. >>> s = "GATTACA" >>> s[3] = "C" Traceback (most recent call last): File "<stdin>", line 1, in ? TypeError: object doesn't support item assignment >>> s = s[:3] + "C" + s[4:] >>> s 'GATCACA' >>> s = s.replace("G","U") 'UATCACA'

Strings are immutable String methods do not modify the string; they return a new string. >>> sequence = “ACGT” >>> sequence.replace(“A”, “G”) ‘GCGT’ >>> print sequence ACGT >>> new_sequence = sequence.replace(“A”, “G”) >>> print new_sequence GCGT

String summary Methods: S.upper() S.lower() S.count(substring) Basic string operations: S = "AATTGG" # assignment - or use single quotes ' ' s1 + s2 # concatenate s2 * 3 # repeat string s2[i] # index character at position 'i' s2[x:y] # index a substring len(S) # get length of string int(S) # turn a string into an integer float(S) # … or a floating point decimal Methods: S.upper() S.lower() S.count(substring) S.replace(old,new) S.find(substring) S.startswith(substring) S. endswith(substring) Printing: print var1,var2,var3 # print multiple variables print "text",var1,"text” # print a combination of strings and variables

First get it working just for uppercase letters. Sample problem #1 Write a program called dna2rna.py that reads a DNA sequence from the first command line argument, and then prints it as an RNA sequence. Make sure it works for both uppercase and lowercase input. > python dna2rna.py AGTCAGT ACUCAGU > python dna2rna.py actcagt acucagu > python dna2rna.py ACTCagt ACUCagu First get it working just for uppercase letters.

Two solutions import sys sequence = sys.argv[1] new_sequence = sequence.replace(“T”, “U”) newer_sequence = new_sequence.replace(“t”, “u”) print newer_sequence print sys.argv[1]

Two solutions import sys sequence = sys.argv[1] new_sequence = sequence.replace(“T”, “U”) newer_sequence = new_sequence.replace(“t”, “u”) print newer_sequence print sys.argv[1].replace(“T”, “U”)

Two solutions import sys sequence = sys.argv[1] new_sequence = sequence.replace(“T”, “U”) newer_sequence = new_sequence.replace(“t”, “u”) print newer_sequence print sys.argv[1].replace(“T”, “U”).replace(“t”, “u”) It is legal (but not always desirable) to chain together multiple methods on a single line.

Sample problem #2 Write a program get-codons.py that reads the first command line argument as a DNA sequence and prints the first three codons, one per line, in uppercase letters. > python get-codons.py TTGCAGTCG TTG CAG TCG > python get-codons.py TTGCAGTCGATC > python get-codons.py tcgatcgac ATC GAC

Solution #2 import sys sequence = sys.argv[1] upper_sequence = sequence.upper() print upper_sequence[:3] print upper_sequence[3:6] print upper_sequence[6:9]

Reading Chapter 1-2 of Think Python (1st edition) by Allen B. Downey.